It is desirable, when grading subsurfaces for superspeed highways, airport runways, and the like that the grade as well as cross slope approximate the finished surface as closely as possible. This procedure ensures a finished paved mat of uniform thickness and strength. In the past the subsurface has been graded by motor graders which were operated by skilled operators having the capability of finishing a surface which would be quite acceptable. However, with increased mechanization and the increased demand for uniform surfaces laid to close tolerances, the "eyeball" method has proven to be unsatisfactory in many instances.
It is well known in the art to use a control console in the cab of a motor grader with means for presetting the slope of the blade and maintaining that slope by servo valves or the like actuated by a pendulum apparatus or its equivalent. The slope sensing mechanism for such systems may be mounted on the blade itself or on some part of the machine structure which is in fixed position relative to the blade. Although such systems may be made to accurately reflect the slope of the blade at all times relative to the horizontal, they do not always reflect the true "cross slope" of the plane being cut, i.e., the slope normal to the direction of travel of the vehicle. Significant error occurs when the blade is rotated away from a position perpendicular to the direction of travel, which is normally the case during operation.
It is likewise known in the art to have sensing means at one end of the blade including a wheel, skid or other means for detecting a predetermined grade datum including means, such as servo valves, to maintain one end of the blade in a working position in accord with any discrepancy signal received from the detecting means. The slope of the blade is then slaved to the grade controlled end. If desired, both ends of the blade could, of course, be controlled by some preset grade datum.
Although the use of slope and grade sensors has done a great deal to automate the road grader when working from a preset datum, such controls are not completely accurate. The grader blade is normally mounted on a turntable or ring-shaped circle, and the width of the swath of the blade is determined by the relative angle of the blade to the direction of movement of the grader. Some amount of angle on the blade is normally maintained for the purpose of moving material to one side of the blade and away from the cutting edge. Once a slope angle is determined and the blade set to this angle, if the operator were to narrow or widen the swath by rotating the turntable or "circle" and the attached blade the effective cross slope angle of the cutting edge of the blade and therefore the resultant cross slope of the graded surface, i.e., perpendicular to the direction of travel, is changed.
With the above-noted problems in mind, it is an object of the present invention to provide a control system for a motor grader or the like which assures that the cutting edge of the blade is maintained at the correct slope angle so as to cut at a constant predetermined "cross slope" angle, irrespective of the relative angularity of the blade.
Yet another object of the present invention is to provide a control system for a motor grader including means for maintaining the cutting edge of the blade at a slope angle which gives a constant "cross slope" angle regardless of the blade angle without further adjustment by the operator.
It is yet a further object of the present invention to provide a control system whereby the cutting edge of the blade of a motor grader or the like may be maintained in a constant preset datum plane, at the desired grade, requiring the operator only to steer the vehicle and control the rotary position of the grader blade.